So what is the fastest process then ?
Not detailed in my original post but here is the goal of all this.
I had a single server initially setup with 5x2TB.
When the first drive failed, I replaced with 4TB in order to get a a pool expansion after fully upgrading to 4TB drives.
But I got myself a second server I want to use for backup so I decided to reduce the pool to 4x4TB and reuse the 2TB in the new TN server.
Hence, I wanted to make a full copy of the actual pool, before destroying the 5X and creating the 4X.
Once the new 4X pool is created, I will have to repeat the process of getting the data from the backup to the main server so if any process is faster, I am all ears.